The research proposed in this application is designed to investigate the mechanisms of measles virus latency in infected cells. Both in vitro as well as in vivo techniques will be utilized to accomplish the specific aims of the proposal. The specific aims are as follows: (1) The isolation and characterization of temperature sensitive mutants of measles virus. (2) The study of the relationship between temperature sensitivity and neurovirulence of measles virus. (3) The study of neuropathology after infection with temperature sensitive mutants of measles virus. (4) The involvement of temperature sensitivity in the induction and maintenance of virus latency. (5) The extension of studies involving the mechanism of measles virus latency in hamster embryo fibroblast cells. The achievement of these goals will yield important knowledge which can be used for the understanding of latent virus infections and the relationship of such interactions to human disease, including the neurological disease, multiple sclerosis and subacute sclerosing panencephalitis.